Plants can assimilate soil ammonia or nitrate reduced to ammonia into organic form in leaves or roots. Ammonia assimilation into glutamine and glutamate occurs primarily in leaf chloroplasts or in root plastids by the combined action of chloroplast glutamine synthetase (GS2; GLN2 gene) and glutamate synthase (GOGAT) (Miflin, B. J. & Lea, P. J., 1977, Ann. Rev. Plant Physiol. 28:299-329). As the assimilation of inorganic nitrogen into organic form requires carbon skeletons, reducing equivalents, and ATP, light serves to coordinate nitrogen assimilation with photosynthesis. Genes involved in plant nitrogen assimilation are induced directly by light (via phytochrome), as well as indirectly by metabolic changes in photosynthate. For example, it has been shown that sucrose supplementation to plant growth media can at least partially induce the expression of mRNA for GLN2 or nitrate reductase (NR) in the absence of light (Cheng et al., 1992, Proc. Natl. Acad. Sci. USA. 89:1861-1864; Faure et al., 1994, Plant J. 5:481-491). Conversely, sucrose can repress the expression of asparagine synthetase (ASN1) (Lam et al., 1994, Plant Physiol. 106:1347-1357). More recently, it has been shown that the effects of sucrose on gene expression can be reversed by the addition of an organic nitrogen source both for nitrate reductase (NR) (Vincentz et al., 1993, Plant J. 3:315-324) and for ASN1 (Lam et al., 1994, Plant Physiol. 106:1347-1357). These findings indicate that plants are able to sense levels of carbon and organic nitrogen, and in turn modulate the expression of genes involved in nitrogen assimilation.
Bacteria can also assimilate ammonia into glutamate or glutamine. Plants' ability to sense changes in the levels of carbon and nitrogen metabolites is reminiscent of a nitrogen regulatory system (Ntr) in bacteria in which a protein called PII, encoded by the glnB gene, can regulate the assimilation of nitrogen into glutamine via glutamine synthetase (CS; glnA) in response to changes in the ratio of organic nitrogen to carbon metabolites (Magasanik, B., 1994, J. Cell. Biochem. 51:34-40).
In response to changes in the metabolic status (i.e., ratio of glutamine to .alpha.-ketoglutarate [gln/.alpha.-KG]), the PII protein of bacteria interacts with a set of partners to regulate the glnA gene at the transcriptional level, and to regulate GS enzyme activity at the post-translational level (Magasanik, B., 1994, J. Cell. Biochem. 51:34-40). Changes in the gln/.alpha.-KG ratio affect the activity of the PII protein via a post-translational modification (uridylylation) at Tyr51 (Magasanik, B., 1988, TIBS 13:475-479). In response to low gln/.alpha.-KG, the PII protein is uridylylated by uridylyltransferase (UTase) (id.). The PII-UMP thus formed then interacts with an adenylyltransferase (ATase) to deadenylylate the GS-AMP enzyme and thereby activate the GS enzyme (Magasanik, B., 1994, J. Cell. Biochem. 51:34-40). A high gln/.alpha.-KC ratio causes the deuridylylation of PII-UMP. This unmodified form of PII interacts with ATase to stimulate the adenylylation and inactivation of the GS enzyme. The ability of ATase to attach or remove AMP from the GS enzyme is dependent on the interaction of ATase with PII or PII-UMP, respectively (Foor et al., 1975, Proc. Natl. Acad. Sci. USA 72:4844-4848). Thus, the nitrogen-regulatory protein PII, is a signal trarsducer whose post-translational modification indirectly regulates GS enzyme activity post-translationally. In addition to its ability to regulate the GS holoenzyme activity, PII can also interact with a two-component system (NRII/NRI, or NtrB/NtrC) to regulate the transcription of the glnA gene encoding GS (Magasanik, B., 1994, J. Cell. Biochem. 51:34-40). Under low gln/.alpha.-KG levels, NRII-kinase phosphorylates NRI which then interacts with the .sigma..sup.54 to activate glnA gene expression (Ninfa, A. J. and Magasanik, B., 1986, Proc. Natl. Acad. Sci. USA 83:5909-5913). When the gln/.alpha.-KG ratio is high, the interaction of PII with NRII stimulates the NRII-phosphatase activity to dephosphorylate NRI-phosphate, and turn off the inducible promoter of glnA transcription (Ninfa, A. J. and Magasanik, B., 1986, Proc. Natl. Acad. Sci. USA 83:5909-5913). Thus, the nitrogen-regulatory protein PII works in concert with other proteins, including UTase, ATase, NRII, and NRI, to regulate glutamine synthetase enzyme activity or glnA transcription in response to the ratio of organic nitrogen to carbon metabolites (Magasanik, B., 1994, J. Cell. Biochem. 51:34-40).
To date, PII homologues have been identified in a diverse set of bacteria including enteric bacteria (Magasanik, B., 1994, J. Cell. Biochem. 51:34-40), cyanobacteria (Tsinoremas et al., 1991, Proc. Natl. Acad. Sci. USA 88:4565-4569), Bacillus (Wray et al., 1994, J. Bacteriol. 176:108-114), and in archaebacteria (Souillard, N. and Sibold, I., 1989, Mol. Microbiol. 3:541-551).